Tag Archives: Delaware

Storm Surges, Global Warming, and Delaware Beaches

ADDENDUM (Nov.-7, 2012): Time lapse video from Delaware Sea Grant.

Rising seas and flood waters cause most of the damage during storms such as Sandy did last week. Tides, waves, and storms all contribute. We can debate how global warming impacts any of the above, but the arguments are involved. So lets assume, that neither tides, waves, nor storms are impacted by global warming, but that the globally averaged rise in sea level over the last 50 or 100 years is. This global warming induced sea level rise is about half a foot in 50 years (3 mm/year), but why would we care about global averages, when we live in Delaware? Furthermore, why worry about the whimpy surges we get ever 2-3 weeks. We don’t, we worry most about the most extreme events like Sandy and want to know how often they occur. Below I show a Sandy-like event to occur about once every 10 years. Furthermore, over time Delaware’s most extreme storm surges are rising twice as fast as global averages do. So, how much does the global warming impact our local flooding in Delaware?

Market Street on the beach in Lewes is in one of the lowest lying areas of town and takes its good old-time draining. This photograph looks northeastward toward the beach, just west of the intersection with Massachusetts Avenue. [Credit: Cape Gazette]

More than I initially thought: the largest storm surge that has hit Delaware was the Ash Wednesday storm on March 6, 1962 which added 5.8 feet to the regular tides and waves. I wrote about this yesterday using public NOAA data. This same storm today would add 6.8 feet to the regular tides and waves. For comparison, Sandy’s storm surge added 5.3 feet. So Sandy was a weak storm by comparison. If it had hit in 1962, it would have added only 4.3 feet. The difference of 1 foot in 50 years is due to steadily rising sea levels:

Largest storm surge at Lewes, Delaware each year from 1957 to present. The red line is a linear fit to the data. The slope indicates that the largest storm surge increase by almost 3 inches every 10 years.

On average each year has a larger largest surge than the year before. While this steady increase by 2.8 +/- 1.7 inches each 10 years is statistically significant (95% confidence), picking the extreme each year is perhaps not the best statistic as extremes do not happen often. Please note that a 95% confidence means that there is a 5% chance that the true increase is either smaller than 0.9 inches/decade or larger than 4.5 inches/decade.

What about the mean or average surge each year? From hourly data, I pick the middle surge, that is, half the surges each year are larger and half are smaller:

This increase of 1.4 +/- 0.2 inches per decade (95% confidence) is more in line of the global average. The uncertainty in this trend is smaller than that of the trend for the extreme, because the median sea level varies little from year to year, while the extreme value varies more from year to year. So, from these results we can conclude, that while the mean or median sea level at Lewes increases by perhaps 1.5 inches in 10 years, the extremes increase twice as fast. So, storm surges like Sandy will become more common than they are today mostly because of global warming.

Over the last 50 years we had at least 5 such events in 1962, 1968, 1996, 1998, and 2012. So, on average we have a Sandy-type storm surge greater than 5 feet every 10 years. This contradicts a Wilmington News Journal article today which quotes John Ramsey to describe “… Sandy as a 1-in-200-years storm, unlikely to be repeated anytime soon. That could give coastal communities time enough to deal with the real threats and realities of sea level rise and climate change.” There is no such time, as it is mis-leading to describe Sandy as a 1-in-200-year event when it has happened about every 10 years during the last 55 years. Instead of a 0.5% percent chance of a Sandy-like event to hit Lewes each year, I would raise this chance to be larger than 10%.

Rising Seas, Storms, and Flooding

Ocean waters are rising and flooding inland waters in Delaware and elsewhere. Some of this is perfectly regular and normal such as the up and down of the tides. Some of it is irregular and normal such as caused by storms, river discharges, waves, and weather. And some is caused by global warming as we continue to burn coal and oil to power our economies. Lets have a quick look at what all this looks like and try to put this into some perspective, but Sandy’s 5.3 feet surge last monday was second to the 5.5 feet surge that hit Lewes in 1962.

Cedar Street in Lewes flooded on Monday, October 29. (Photo by: Don Bland), as published by Cape Gazette.

The up and down of the tides each day is about 3 feet in Lewes, Delaware. This large change in sea level is so regular, normal, and predictable, that I remove it from all further discussion, because I want to know how extreme an event this week’s storm Sandy was. For this purpose I downloaded all the hourly tide gauge data from Lewes, Delaware from NOAA. The record starts in 1957 and is ongoing. Here is how the record looks for the last 4 weeks including the surge caused by Sandy last monday:

Sandy’s storm surge added 5.3 feet to the regular tide which is second-largest surge in the historical record. The largest surge at Lewes, DE was caused by the 1962 Ash Wednesday Storm that added 5.5 feet to the regular sea level:

So while Sandy was a very large surge, it was neither unprecedented nor a once in a century event. Furthermore, and this is where I come back to global warming, the 5.3 feet 2012 surge of Sandy includes the last 50 years of steady sea level rise which comes to about an inch every 10-15 years or about half a foot in the ~50 years between 1962 and 2012. So, a repeat of the 1962 storm system would cause a 6.0 feet and not the 5.5 feet surge that took place in 1962.

Furthermore, while the real size of the surge depends on where the center of the storm makes land-fall or where you are relative to the storm, the rising seas caused by global warming are much more uniform, that is, they are little different in Boston, New York, Lewes, Norfolk, or even San Francisco:

So, global warming and the rising seas it causes are both real and here to stay. Global warming provides the upward creeping background sea level to which larger tides, waves, and surges add. The combined effect of all these cause the coastal flooding. So 50 years from now, a rare, but perhaps perfectly natural freak storm like Sandy will cause a storm surge of 5.8 instead 5.3 feet on account of global warming. About 1/3 of this added sea level is caused by the oceans expanding as they warm, another 1/3 is caused by melting glaciers and ice sheets in Greenland and Antarctica, and the last 1/3 is caused by other processes. So, what happens on Greenland or China does not stay there, it impacts present and future sea level in Lewes, DE.